1. Technical Field
The present invention relates to integrated circuits in general, and in particular to an apparatus for assisting device modification. Still more particularly, the present invention relates to an apparatus for assisting backside focused ion beam device modification.
2. Description of the Prior Art
In order to implement physical changes to an integrated circuit, one can modify the masks used to manufacture an integrate circuit, or one can modify it physically by changing the circuit connections within the integrated circuit itself. Modifying masks and manufacturing a new integrated circuit can be costly and time consuming. Although attempting to rewire or make other edits to the integrate circuit can also be a time consuming process, it offers the chance to verify changes to the integrated circuit in hardware without the delays and costs associated with remanufacturing. Typically, one determines the best approach to modify the circuit by selecting circuits and conductive lines within the integrated circuit that are accessible and allow the desired modification to be made. Then, one must perform cuts and connections to form the necessary interconnections between the selected circuits. Such modifications, or edits, are commonly formed using a focused ion beam (FIB) machine. A FIB machine is capable of adding or removing both conductive and insulative materials in an integrated circuit. Removing conductive material produces an opening, known as an xe2x80x9cinsulation cut.xe2x80x9d Filling a hole with conductive material creates a xe2x80x9cmetal viaxe2x80x9d that can be used to re-route electrical connections. Removal of insulative material is used to access conductors and circuits of interest, whereas deposition of insulative material is used to shield conductors and circuits from unwanted electrical connection.
Nevertheless, editing an integrated circuit with a FIB machine itself can be prohibitive; FIB machine capabilities may not be compatible with advanced semiconductor technology fabrication techniques, geometry, and materials. Typically, one makes FIB modifications from the front or wiring side of the integrated circuit. However, the multi-layer wiring in integrated circuits prevents the necessary FIB modifications from being made since the higher-lying wiring layers obscure access to the wires that are to be modified. An attempt to access the circuits and wires of interest would damage the overlying wiring, degrading circuit performance or even rendering the circuit completely non-functional. Additionally, the dimensions of a FIB cut or deposition, and the extra clearances required for such cuts and depositions may be larger than the dimensions and clearances in the existing circuit. For example, a deposited via with a width of 0.5 microns is too wide to fit between diffusions that are spaced 0.3 microns apart. There are also material constraints associated with the chemistry used in FIB depositions and the resultant physical properties of the FIB-deposited material. Even if there is sufficient access from the front side, and there are no limiting geometrical constraints, the interaction between the ion beam and the chemicals used in the FIB machine may prevent successful FIB editing. For example, a FIB machine can deposit reliably a conductive line having a length of less than 500 microns. The limitation is imposed because of voiding within the deposited conductor and the high resistivity of the resultant line. Another example, is the affect of ion implantation that occurs as a side effect of using a FIB. These ions can alter the electrical properties of the integrated circuit after FIB edits are complete. Finally, there are resolution and accuracy issues associated with navigation and focused ion beam placement that can limit the effectiveness of FIB edits.
Because of the aforementioned constraints, and the growing prevalence of so-called xe2x80x9cflip-chipxe2x80x9d packaging, making FIB edits through the silicon substrate on which the integrated circuit is built becomes important. Such processes in which the edits are made in such a manner are termed xe2x80x9cbackside FIB,xe2x80x9d and the substrate side of the integrated circuit is referred to as the xe2x80x9cbackxe2x80x9d side. However, making FIB edits from the back side is limited by some of the same constraints described for frontside FIB. Specifically, there are similar geometrical constraints associated with the closely spaced wires and diffusions and the inability to deposit material with the necessary physical properties. Furthermore, new circuit technologies including so-called xe2x80x9clow-kxe2x80x9d dielectrics and silicon-on-insulator technology pose other constraints on backside FIB methods and materials. Therefore, new structures and methods are needed to incorporate the capacity for FIB editing in state-of-the-art integrated circuits.
In accordance with a preferred embodiment of the present invention, an apparatus for assisting backside focused ion beam (FIB) device modification circuit includes a FIB control circuit and a FIB device modification circuit. The FIB control circuit, which is coupled to the FIB device modification circuit, contains FIB accessible structures. The FIB control circuit is preferably located in a proximity of a backside of a substrate to allow a FIB machine to edit the structures creating an electrical connection jumper) and/or an insulation cut. The FIB control circuit includes at least one control line for controlling the FIB device modification circuit. The state or control level of the control line is changed when the jumper and/or cut is connected and/or severed by the FIB machine. The FIB device modification circuit includes a control input from the FIB control circuit, an output, and may include an input, a FIB input pad, and a FIB output pad. When the FIB device modification circuit has a control line, an output, and no input, the circuit electrically modifies drive strength on the output. When the FIB device modification circuit has a control line, an input and an output, the circuit allows the input to be electrically connected to the output.
All objects, features, and advantages of the present invention will become apparent in the following detailed written description.